Although color film has only become widely popular in the last 25 or so years, the basic principles are almost as old as photography itself. All color film is based upon the discovery in the mid-19th century that colored light can be split into three primary colors: blue, green and red. If you project each of these three colors separately onto a screen, you can make white and almost any other color in the spectrum by overlapping them in proportion.
Color print film consists of three separate emulsion layers (one sensitive to blue light, one to green and the other to red) all coated together onto a film base. This film "tripack" is generally constructed as follows: a top layer sensitive to blue and ultraviolet light and containing silver bromide without any color sensitization dyes. The second layer is sensitive to both blue and green light and consists of a silver bromide emulsion with certain color sensitization dyes added.
Since this second layer of the film must only record the green component of the subject, blue light is filtered out by a yellow barrier layer between the first and second emulsion layers. The yellow barrier layer is not made from dyes or pigments, but is colored with finely divided colloidal silver particles which can be removed by processing in a "bleach" bath. The final layer of the film, next to the film base, is sensitive to both blue light and to red light. Since the blue light has already been filtered out by the yellow barrier before it reaches this layer, it records only the red component of the subject.
The three emulsion layers, then, each form an image of one of the three primary colors, blue, green and red. Before processing, however, the film is not actually colored. Color is provided by the formation of appropriately colored dyes in each of the three emulsion layers during processing. The formation of the dyes is controlled by complex organic chemicals known as color couplers. Each of the three emulsion layers contains a different color coupler.
When color print film is processed, the developer reacts with grains of silver in the emulsions to form a silver image just like a black and white negative. Some by-products are produced which react with the color couplers to form the dyes that make the color image. Around each grain of silver in the blue sensitive layer, for example, yellow dye is formed and a yellow image appears. The same type of process occurs in the other layers. Where green light has struck the green sensitive layer, a magenta image is formed, and where red light has struck the red sensitive layer, a cyan (blue-green) image is formed.
After color development, there is both a dye image and a silver image in each emulsion layer. The silver is removed with a chemical agent in subsequent processing which converts silver back into silver bromide without affecting the dye images in each layer. This chemical agent however also selectively "bleaches out" the yellow filter layer. After this processing step, the film is immersed in a "fixing" bath which solubilizes the silver bromide so that it can be "washed out" in the next processing step before the film is hung up to dry. The most common color negative process was developed by Eastman Kodak and is called the "C-41 film processing system".
The resultant "color negative" is negative in two senses. Not only are the light tones in the subject reproduced as dark in the negative, but all colors in the subject are reproduced as their complementaries in this negative. Each of the three primary colors are rendered as their three complementary colors: blue becomes yellow, green becomes magenta, and red becomes cyan. Just as the printing of a black and white negative gives the image its original values, so too does the printing of a color negative. Color printing paper works in a similar way to film and the "print" is simply a negative image of the negative, that is, a positive.
In contrast to color print film, color reversal film (also often referred to as color slide film or color transparency film) is designed to make color positive images (color transparencies) when the film is processed by a processing method referred to as Ektachrome Process E-6 or simply shortened to "E-6". Ektachrome is Eastman Kodak's trademark. However, E-6 processing chemicals are available from a number of other manufacturers as well. Color reversal film is frequently referred to as "E-6" film and also is available from several manufacturers.
In the typical color reversal film, the latent image, recorded in the three emulsion layers of the film, is developed in complete darkness by the first developer. This produces a negative image comprised of metallic silver which is similar to an ordinary black and white negative. The action of this developer, however, is much more sophisticated. Ultimately, it is responsible for the overall density of the image as well as governing the formation and reproduction of its colors. A brief two or three minute wash follows the first development. The wash removes excess developer and so prevents development from continuing. The next stage is a reversal bath in which the emulsion become laden with a potentially active reversal agent. This activity is triggered by the color developer which follows. Fogging centers form on all the silver halides which were left unexposed by the camera exposure, and were therefore left untouched by the first developer.
The residual silver halide, which is now developable, is converted to a dye image by the color developer. Color couplers contained within the three emulsion layers react with the color developer to produce the yellow component in one layer, the magenta component in another layer, and the cyan component in the third layer. Dye is formed only in those layers where no metallic silver formed during the first development. If one looks at the film at this stage there is no sign of color. All one sees is dense black because the image dyes are still covered by metallic silver. The function of the subsequent processing stages is to remove the silver formed during the first and second development, leaving behind only dyes which combine to form the color positive image.
By making small adjustments to the first development stage it is possible to obtain acceptable transparencies from films which have been exposed at other than the recommended film speed rating. A film knowingly underexposed by one f-stop (ie., exposed as if it were a faster film) can be rescued by increasing the time in the first development stage, e.g., from six to ten minutes (often referred to as "pushing one f-stop"); two f-stops underexposure requires developing an additional minute and a half ("pushing two f-stops"). The limit of correction in the case of overexposure is about one f-stop, achieved by reducing the first development to four minutes (referred to as "pulling one f-stop"). In effect, for film exposed at the manufacturers' recommended film speed rating, "pushing the film one or two stops" in developing tends to make the resulting color positive transparency film appear slightly overexposed. Likewise, by processing the same film by "pulling one stop" would give the resulting color positive transparency film a slightly underexposed appearance.
Normal processing of color print film (the type of film most frequently used by both amateurs and professionals to make color photographic prints) is generally referred to as "C-41 processing". When color print film is processed by the C-41 film processing method, the resulting product is referred to as "color negative film". This color negative film can then by used directly in typical color printing procedures to produce a color photographic print (positive image) which shows the colors as one would have observed them in the original subject matter. If one processes color reversal film by using a C-41 film processing method, then the resulting image is a also color negative image instead of the color positive image which results from the normally recommended E-6 processing.
Further background material relating to color photographic film, development and processing thereof is hereby incorporated by reference:
Reference 1) "Understanding Color Negative Film", The Photo, Vol. 2, pp. 546-547, Marshall Cavendish Corporation, New York (1986). PA1 Reference 2) "Understanding Color Slide Flmn", The Photo, Vol. 3, pp. 574-575, Marshall Cavendish Corporation, New York (1986). PA1 Reference 3) "Processing Color Slides", The Photo, Vol. 4, pp. 894-895, Marshall Cavendish Corporation, New York (1986). PA1 Reference 4) "Colour Photography", Encyclopedia Britannica, Vol. 25, pp. 790-792 (1991). PA1 Reference 5) "Understanding Solarization and Sabattier Effects", The Photo, Vol. 3, pp. 754-755, Marshall Cavendish Corporation, New York (1986). PA1 Reference 6) "Simple Sabattier Effects", The Photo, Vol. 5, pp. 1358-1361, Marshall Cavendish Corporation, New York (1986). PA1 Reference 7) "Solarization and the Sabattier Effect", Photography in Focus, M. Jacobs and K. Kokrada, pp. 202-205, National Textbook Company, Lincolnwood, PA1 Reference 8) The Book of Special Effects Photography, Michael Langford, Alfred A. Knopf, New York, N.Y. (1981). PA1 Reference 9) "Sandwiching", Outstanding Special Effects Photography on a Limited Budget, Jim Zuckerman, Chapter 5, pp. 63-75, Writer's Digest Books, Cincinnati, Ohio (1993).
Of all the techniques for creating special effects in the darkroom, few can transform an image more dramatically than those based upon a phenomenon discovered in 1862 by a Frenchman, Armand Sabattier. Sabattier noticed that if a wet collodion plate negative was exposed to light during development, the image was partially reversed and became a positive. Modern photographic film and photographic paper can be reversed similarly, by exposure to light during development. This phenomenon, now known as the "Sabattier effect", is often confused with a similar reversal effect known as "solarization", and indeed, the technique for using the Sabattier effect is sometimes known as "pseudo-solarization ". The Sabattier effect and solarization, in effect, produce both a negative and positive image on the same film or paper.
A number of theories have been put forward to explain the Sabattier effect. The most accepted explanation is that the first image that appears on the paper or film forms a mask or stencil. The metallic silver in the shadow areas absorbs much of the light from the second exposure and when white light is turned on, these areas are actually less heavily exposed. The result is that the highlights darken more quickly than the shadows.
An overall fogging exposure by light in the middle of development produces the solarization effect which is characterized by darkening the previously undeveloped areas and reversing some of the color tones. Although solarization and the Sabattier effect look similar, solarization takes place for quite different reasons. Unlike the Sabattier effect, which can only take place during development, solarization is due to excessive exposure at any time. True solarization is very difficult to achieve so photographers generally employ the Sabattier effect since they can get much the same result. The process which involves exposing a print to light during development is relatively uncertain so photographers must do a lot of experimentation on any one photographic print and they find the process very difficult to repeat.
Because of the difficulty in reproducing the Sabattier effect or solarization, it is generally done in black and white since it is less complex than color. However, the Sabattier effect or solarization is even more startling in color. As expected, the results are even less predictable in color than in black and white and the failure rate is very high since there are three emulsion layers in color film, not just one as in black and white. It is possible to solarize color reversal film, unfortunately, the process does not work very well with reversal film material and the cost is generally considered too prohibitive for the amount of experimenting that is needed to find the best combination of development times and fogging exposures (see reference 6 below).
Further background material relating to the Sabattier effect (pseudosolarization) and color solarization is hereby incorporated by reference:
Ill. (1985).
My invention produces an effect which has characteristics similar to that produced by color solarization or the Sabattier Effect. However, with my invention color positive film images can be produced with a high degree of reproducibility, made with a broad range of color possibilities and a great degree of color density control. Moreover, my invention permits an artist/photographer to create a measurable and calculatable (by f-stops) degree of contrast in the resultant composite image. The broad range of color flexibility and greater degree of control greatly reduces the time and cost in producing a color positive film image with a "solarization effect" suitable for commercial use in cinematography, signs or for the production of photographic prints or conventional photomechanical reproduction processes for advertising brochures and magazine or book illustrations.
Photographers frequently sandwich (layer or laminate) two photographic film images to produce a photographic montage, which is a combination of two or more different images taken at different times. They may sandwich two different color positive transparency images taken of different subjects exposed on color reversal film which is subsequently processed using the recommended E-6 film processing method. Also, they may sandwich two different color negatives taken of different subjects at different times to achieve a photographic montage effect. In some instances, photographers have sandwiched color positive transparencies with black and white negatives of the same image or black and white positive images with color negatives of the same or substantially the same image (See Reference 8, p 112).
Photographers are unable, in practice, to sandwich (layer or laminate) color positive image transparency film in a registered or even near-registered manner with a color negative image transparency film taken of the same subject, or substantially the same subject, since color theory would predict that the combined image would appear substantially black or opaque in as much as the complementary colors in the respective positive and negative images would effectively filter out or absorb most, if not all, of the transmittable white light. Further background information relating to sandwich structures is hereby incorporated by reference:
Therefore, it is unexpected and novel to sandwich (layer or laminate) color positive image transparency film with color negative image transparency film capturing the same, or substantially the same, subject matter as in the present invention for the purpose of producing a color photographic transparency film laminate for use in projecting the resulting image, for cinematographic purposes, for subsequent production of a photographic print therefrom, for photomechanical reproduction using offset printing or silkscreen printing.